Current Density of the Arc Cathode Spot

New measurements of the area of the arc cathode spot from measurements of width of tracks left on metals by a cathode spot moving in a magnetic field indicate that the current density is greater than values given by previous workers. At 2.6 amperes, we find the following values in air at atmospheric pressure: Cu 124,000 amp./${\mathrm{cm}}^2$, Al 29,500 amp./${\mathrm{cm}}^2$, W 74,000 amp./${\mathrm{cm}}^2$, Hg on Cu 20,700 amp./${\mathrm{cm}}^2$.